This invention relates to the addition of polyamides to crystalline polymers pigmented with organic pigments, to a process for the production of crystalline polymers, to injection-molded articles with improved performance properties produced therefrom and to preparations containing polyamides and organic pigments.
Crystalline polymers, such as in particular high-density polyethylene, are widely used for the production of injection-molded articles, such as for example plastic casks, crates, bottles or garden furniture. To this end, the melted polymer is injected into the mold, generally at around 300.degree. C., the mold is then gradually cooled and, finally, the injection molding is removed from the mold. In many cases, it is desirable to produce colored products so that pigments are added to the polymer melts. Phthalocyanines, for example, are used to produce intensive blue and green tones while azo dyes or anthraquinone dyes, for example, are used for red and orange tones. However, a disadvantage in this regard is that, above a critical concentration of around 50 ppm, these organic dyes are capable of interacting with the polymers so that the crystal structure is disturbed. This is reflected in deformation (so-called warpage) of the injection-molded articles, i.e. a surface which, in principle, ought actually to have been flat corresponding to the injection mold develops undulations instead, with the result that, for example, the products are very difficult to stack. Another major disadvantage is that the crystal structure is occasionally disturbed so seriously that cracks and fractures can form in the material on cooling. It will readily be appreciated that, for example, a plastic cask which develops cracks after storage overnight outdoors has no further practical use.
There has been no shortage of attempts in the past to remedy this problem. However, these attempts largely come down to the use of inorganic pigments as opposed to organic pigments. Unfortunately, this seriously restricts the color spectrum. Another possible solution is to allow the injection-molded articles to cool over long periods in the mold. This does not of course help the throughput. German patent application DE-A1 4313090 (BASF) describes a process for avoiding warpage effects in injection-molded articles of HDPE which is characterized by the use of organic pigments which have been subjected to a low-temperature plasma treatment. This process is clearly unsuitable for operation on an industrial scale on account of the large amount of equipment involved. In addition, the crystallinity of the polymer can be reduced by co-condensing corresponding comonomers such as, for example, polypropylene or maleic anhydride [JP-A Hei 06/329811 (Showa Denko KK); U.S. Pat. No. 4,032,600 (Chemplex)]. However, polymers of this type do not necessarily satisfy the requirements involved.
Accordingly, the complex problem addressed by the present invention was to provide additives which would stabilize crystalline polymers pigmented with organic pigments, preferably HDPE pigmented with phthalocyanines, in such a way that warpaging and cold fractures no longer occur.